The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.
Not applicable.
(1) Field of the Invention
The present invention relates generally to direct measurement of phase voltage and current parameters and, more specifically, to apparatus and method for measurement of individual phase power for unbalanced three-phase delta power configurations.
(2) Description of the Prior Art
In a submarine as well as in many commercial power configurations, three-phase power may be supplied only in a delta configuration. The measurement of individual phase power for unbalanced three phase delta loads requires that the individual phase voltage and current be accessible. Because delta power offers only phase voltage and line current, the required phase current is typically not available. An exception is the case where the load is made up of three sets of single phase loads and both conductors for each single-phase load are accessible. In the more typical situation where only phase voltage and line current are available, the determination of phase current given the line current is mathematically not possible due to unknown contributions of phase currents that form the resulting line currents. Moreover, in an unbalanced system the individual phases are not necessarily at a predetermined magnitude and phase relationship with each other thereby making presumptions based on incomplete information likely to be inaccurate.
It would be desirable to allow for direct measurement of unbalanced individual phase power and power factor for delta loads without internal intrusion into either the load or the source. Moreover, it would be desirable to provide a means for such direct measurement using commercially available components without the need for specially made elements.
Several of the following patents disclose prior art efforts to provide for power measurement systems but do not disclose a solution to the above problems.
U.S. Pat. No. 5,631,818, issued May 20, 1997 to Johnson et al, discloses a power supply system particularly adapted for use with an electrostatic precipitator providing substantially ripple-free DC power for improved precipitator operation. The power supply is adapted to receive three-phase AC power and to transform the AC power into high voltage DC power having a minimum of voltage ripple in the output. The power supply includes a multi-phase transformer having three primary windings, each of the primary windings having associated with it a pair of secondary windings. The primary windings can be either delta connected or wye connected. One of each of the secondary windings associated with one of the respective primary windings are connected together in a delta connection arrangement, and the remaining secondary windings are connected in a wye connection arrangement. Because the respective AC voltage outputs of each of the secondary windings is out of phase with the AC voltage outputs of the other secondary windings, the resultant combined DC output voltage, after the DC voltages have been rectified in a three phase, full-wave bridge rectifier stack, produces minimal ripple voltage and current, without requiring additional, more expensive, and less reliable components.
U.S. Pat. No. 4,896,106 issued Jan. 23, 1990 to Voisine and Gausegger, discloses a watt-hour meter, for a wye connected load, and includes a current transformer for sensing the load current, a resistive voltage divider network for sensing the line voltage, and a measuring circuit for calculating the AC electrical energy consumption of the load based on the sensed load current and line voltage. The load consumption calculations are performed using a Mark-Space oscillator and amplitude modulator in the watt transducer chip. Reduced voltage from the resistive voltage divider network is fed into the input junction of the Mark-Space oscillator. A feedback resistor is connected from the output junction to the input junction of the oscillator. The feedback resistor and the voltage divider network are mounted on the same thermally conductive substrate so that each is subject to the same thermal effects. Thus, any change, due to thermal effects, in the current through the voltage divider network seen at the input junction to the Mark-Space oscillator are compensated by a current change through the feedback line due to thermal effects on the feedback resistor. The watt-hour meter also includes an electronic register that receives the load consumption information from the watt transducer chip. The register is electrically isolated from the watt transducer chip and, therefore, from the voltage diver network, by an opto-isolator to reduce the possibility of electrical shock at the register. Power is provided to the watt transducer chip and the electronic register from the split secondary coils of a power transformer.
U.S. Pat. No. 3,944,919 issued Mar. 16, 1976 to Jewell et al, discloses a DC measuring circuit for power rectifiers wherein primary line current is utilized to measure direct current output. In the measuring circuit, line current transformers are connected to a multiphase instrument rectifier through a phase shifting current transformer which is adjustable to compensate for difference between the primary to secondary line current phase shift characteristic of the power transformer and any phase shift in the line current transformers. By such compensation, the average value of the rectified primary line current in the measuring circuit is maintained independent of the commutating angle of the rectified power current.
U.S. Pat. No. 5,691,634, issued Nov. 25, 1997, to C. S. Lowe, discloses a device for monitoring power in a wye power service having three phases including a sampling element arranged and configured to sample a current value and a voltage value from each of the three phases. A processor, coupled to said sampling element, calculates power either by using the current value and the voltage value for the phase when the voltage for that phase is available, or by using the current value for at least one phase and the voltage values for the other two phases.
U.S. Pat. No. 2,966,629, issued Dec. 27, 1960, to Downing et al., discloses a circuit for measuring an indication of total apparent power from a balanced system in which a delta transformer and a three-wire current sensing transformer are coupled between the source and load.
U.S. Pat. No. 2,071,834, issued Feb. 23, 1937, to E. L. Harder, discloses a wye-delta transformer for use with respect to alternating current impedance drop compensators.
In summary, while the above listed prior art shows various systems and power measurement means, the prior art does not show an apparatus or method for determining the individual phase currents provided from a delta source to a delta load or the means to determine individual phase powers and power factors.
Consequently, there remains a need for a system that allows for measurements of individual phase powers using commercially available components. Those skilled in the art will appreciate the present invention that addresses the above and other problems.
Accordingly, it is an object of the present invention to provide an apparatus and method for determining equivalent individual phase powers from an unbalanced delta power configuration.
It is another object of the present invention to provide apparatus and methods for directly measuring individual equivalent phase voltage and phase current from a delta power configuration.
It is yet another object of the present invention to use commercially available components to meet the above objects.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims.
An apparatus is disclosed for measuring equivalent individual phase voltages and phase currents from an alternating delta power source having a first delta source, a second delta source, and a third delta source. The delta power source provides power for use by a delta load circuit having a first delta load, a second delta load, and a third delta load. The apparatus comprises an input delta-wye transformer having a first delta input coil, a second delta input coil, a third delta input coil, a first wye input coil with a first wye input connection, a second wye input coil with a second wye input connection and a point connection electrically connects the first wye input coil and the second wye input coil and the third wye input coil. The first wye input coil may be inductively coupled to the first delta input coil, the second wye input coil is inductively coupled to the second delta input coil, and the third wye input coil may be inductively coupled to the third delta input coil. The first delta input coil is connected across the first delta source, the second delta input coil across the second delta source, and the third delta input coil across the third delta source.
An output wye-delta transformer has a first wye output coil with a first wye output connection, a second wye output coil with a second wye output connection, a third wye output coil with a third wye output connection, a first delta output coil, a second delta output coil, and a third delta output coil. The first wye output coil is inductively coupled to the first delta output coil, the second wye output coil is inductively coupled to the second delta output coil, the third wye output coil is inductively coupled to the third delta output coil. The first delta output coil is connected across the first delta load, the second delta output coil across the second delta load, and the third delta output coil across the third delta load.
A first current conductor, a second current conductor, and a third current conductor are provided. The first current conductor electrically connects the first wye input connection and the first wye output connection. The second current conductor electrically connects the second wye input connection and the second wye output connection. The third current conductor electrically connects the third wye input connection and the third wye output connection.
A first current probe is used for measuring current through the first current conductor connecting the input wye to the output wye, a second current probe for measuring current through the second current conductor connecting the input wye to the output wye, and a third current probe for measuring current through the third current conductor connecting the input wye to the output wye.
A first voltage probe, a second voltage probe, and a third voltage probe are provided for preferably measuring a first voltage and a second voltage and a third voltage related to at least one of the input delta-wye transformer or the output wye-delta transformer.
In other words, a first set of coils is connected in a delta configuration wherein the first set of coils comprises a first delta output coil, a second delta output coil, and a third delta output coil. A second set of coils is connected in a wye configuration wherein the second set of coils comprises a first wye input coil with a first wye input connection, a second wye input coil with a second wye input connection and a third wye input coil with third wye input connection. A third set of coils is connected in a wye configuration wherein the third set of coils comprises a first wye output coil with a first wye output connection, a second wye output coil with a second wye output connection, and a third wye output coil with a third wye output connection. A fourth set of coils is connected in a delta configuration wherein the fourth set of coils comprising a first delta output coil, a second delta output coil, and a third delta output coil. A first current conductor, a second current conductor, and a third current conductor are provided. The first current conductor electrically connects the first wye input connection and the first wye output connection. The second current conductor electrically connects the second wye input connection and the second wye output connection. The third current conductor electrically connects the third wye input connection and the third wye output connection. As before, the first current probe measures current through the first current conductor, the second current probe measures current through the second current conductor, and a third current probe measures current through the third current conductor.
In operation, a method is provided for measuring individual phase voltages and phase currents. Using the equipment described hereinbefore, a first phase current is measured through the first current conductor. A second phase current is measured through the second current conductor. A third phase current is measured through the third current conductor. A first phase voltage is preferably measured from the input delta-wye transformer across the first input wye coil. A second phase voltage is preferably measured from the input delta-wye transformer, across the second input wye coil. A third phase voltage is preferably measured from the input delta-wye transformer, across the third input wye coil. If desired, the phase powers can be determined from the phase voltage and current parameters. A first phase power is determined from the first phase current and the first phase voltage. A second phase power is determined from the second phase current and the second phase voltage. A third phase power is determined from the third phase current and third phase voltage. A total power may be determined from the first phase power and the second phase power and the third phase power. Moreover, a first phase power factor may be determined from the first phase current and the first phase voltage. A second phase power factor may be determined from the second phase current and the second phase voltage. A third phase power factor may be determined from the third phase current and third phase voltage.